A subterranean formation can be drilled with a drill string having a drill bit located on a distal end of the drillstring. A motor can be operatively coupled to rotate the drill bit. During drilling, the rate of penetration (ROP) may be used to measure how quickly the drill bit penetrates the formation. While several factors influence the ROP, the ROP primarily depends on the type of formation being drilled, the weight on bit (WOB), and the rotational speed (revolutions per minute (RPM)) of the drill bit. As the type of formation being drilled is predetermined, operators may vary the WOB and select a downhole motor with appropriate RPM capabilities to impact the ROP during operation.
The relationship between the WOB and RPM for a particular model and size of drilling motor can be represented by a power curve. The power curve is used to determine the energy delivered to the bit for a given WOB and RPM. Although other factors, for example, torque, vibration, fluid rheology, and other features of the fluid, may affect the power curve (as well as the ROP) WOB and RPM play stronger roles in defining a power curve for a given motor.
The WOB can be adjusted at the drill rig by putting more weight on the drill bit or carrying more of the weight of the drill string on the drill rig. However, adjusting the RPM during operation is not as straightforward. As noted above, drilling motors drive the rotation of the drill bit and determine the RPM of the drill bit. Motors, e.g., mud motors, may be driven by drilling fluid pumped from surface equipment through the drillstring. The volume of fluid supplied to the mud motor is correlated to the speed, i.e., RPM of the motor. For example, a higher flow rate of fluid provided to the motor will generally result in a greater RPM of the drill bit. However, the flow rate and RPMs of the motor generally have a parabolic relationship such that a range of peak efficiency for each motor exists, beyond which, providing greater fluid flow does not result in increased RPMs and may result in damage to the motor and/or bit. Similarly, too little fluid flow can stall a motor.
Additionally, drilling fluid provided to the drill string is used to clean away drill cuttings that accumulate in an annular space (“annulus”) between the drill string, including the bottom hole assembly (BHA), and the wall of the borehole. When excessive amounts of cuttings build up in the annulus, the friction on the drill string increases with a corresponding increase of risk of the drill string becoming stuck in the borehole. In general, there is a minimum fluid flow necessary to effectively transport the cuttings up and out of the borehole. To prevent the drill string from becoming stuck, the ROP may be reduced until the excess cuttings are cleared away by the mud flow. In some cases, the flow of drilling fluid provided to the drill string is determined by the cleaning needs of the borehole rather than the RPM of the motor.